A process for producing useful carboxylic acid esters industrially, in the case of methyl methacrylate, for example, may include a process in which methacrylic acid is produced by oxidizing methacrolein with oxygen followed by reacting the methacrylic acid with methanol to product methyl methacrylate. However, the heteropolyacid catalyst used in a step for obtaining methacrylic acid by oxidizing methacrolein has problems with thermal stability, and gradually decomposes under reaction temperature conditions. In addition, the yield is also still not adequate, thus leaving room for improvement as an industrial catalyst.
On the other hand, the direct metha process for producing methyl methacrylate or methyl acrylate in a single step by reacting methacrolein or acrolein with methanol and molecular oxygen is a simple process that does not require separation of easily polymerizable methacrylic acid or acrylic acid, and is currently attracting attention because of its higher yield of methyl methacrylate in comparison with the above process.
In such a process, a catalyst primarily containing palladium is used for the catalyst. However, during production of methyl methacrylate or methyl acrylate in a single step by reacting methacrolein or acrolein with methanol and molecular oxygen, since the methacrolein or acrolein is an unsaturated aldehyde, numerous acetals of the unsaturated aldehyde and alkoxy forms, resulting from addition of alcohol to the unsaturated bonds, are formed as by-products, while also resulting in the problem of generation of carbon dioxide gas which is the final oxidation product (see Patent document 1).
Therefore, modifications have been made to the catalyst to overcome these problems. For example, the above problems regarding the formation of the by-products have been reported to be solved and carboxylic acid ester has been reported to be able to be produced at high yield by using a catalyst containing an intermetallic compound containing palladium and at least one element selected from the group consisting of lead, mercury, bismuth and thallium, or a catalyst containing an alkaline metal compound or alkaline earth metal compound (see Patent Document 2).
On the other hand, although the presence of a catalyst containing palladium was long thought to be required for catalysts used in this process, more recently, catalysts have been reported to have been used that comprise the loading of a noble metal such as ruthenium or gold on a support. Specific examples of such processes include the use of a catalyst in which gold is loaded onto a support (see Patent document 3) or the use of a catalyst comprised of ruthenium (see Patent document 4) during production of carboxylic acid ester by reacting aldehyde and alcohol in the presence of an oxygen-containing gas.    Patent document 1: Japanese Patent Publication No. S45-34368    Patent document 2: Japanese Patent Publication No. S62-7902    Patent document 3: Japanese Patent Application Laid-open No. 2000-154164    Patent document 4: Japanese Patent Application Laid-open No. 2001-220367